In a satellite communication system, a station, usually on the ground, transmits a data stream to the payload of the satellite which transmits the data to one or more recipients on the ground using one or more beams. The data are structured in the form of frames, notably to obtain a sharing of the link, for example between a number of users. In this context, a frame is a block of data to be transmitted at a precise instant, on a determined carrier frequency and in a given beam. Also, when the data stream is not organized according to a distribution law that is known in advance and invariable, but contains frames for which the instant of transmission, the destination carrier frequency and/or the destination beam cannot be predicted, the satellite has to dynamically switch the frames. In other words, the satellite must, after having received the frames, determine in real time their transmission parameters and direct them to the appropriate beam.
A first known technique for dynamically switching frames in the satellite is to use a “remote control” type link. A control centre, sometimes referred to by the acronym MCC standing for “Mission Control Centre” transmits the uplink and downlink time/frequency mapping plans to the satellite, which enables the payload to know the frequencies of the carriers, the switching instants and the destination of the frames. However, this technique requires the transmission of a large volume of control data, which is now always in line with the bit rate constraints available on the satellite. This drawback is all the more critical when the switching parameter refresh period is small, or, in other words, when the “dynamism” of the switching is great.
According to a second possible technique, one or more carriers are dedicated to the signalling. In other words, in addition to the primary carrier or carriers which convey the primary signals, a ground station transmits a modulated carrier, on another frequency, dedicated to transporting signalling data. This dedicated carrier is synchronized with the first carrier, and is demodulated by the payload of the satellite to indicate the frequency of the carrier to which the switching must be performed, the switching instants and the destination of the frames. Furthermore, the dedicated carrier may be shared between a number of users according to a TDMA (Time Division Multiple Access) type multiplexing. However, this second technique requires the occupancy of a portion of the frequency spectrum for the signalling data. Furthermore, the number of carriers or the size of the frequency channel occupied by the signalling depends on the number of ground stations in the network.
According to a third technique which can be envisaged, headers are inserted into the frames of the first signals. The payload of the satellite detects these headers which enable the payload to know the switching instants and the destination of the frames. However, this third technique entails modifying an existing, and therefore potentially standardized, wave form by inserting a specific header or by modifying pre-existing header fields (for example, in the case of ATM-type addressing, ATM standing for “Asynchronous Transfer Mode”). Also, this third technique does not allow (or allows only with difficulty) for a transparent switching of the first signals.